Semiconductor wafers are used for a variety of purposes in the semiconductor, solar energy and other industries. Wafer quality often depends on variables such as thickness and surface characteristics. Poor quality wafers may have uneven thickness or uneven surface characteristics; whereas, higher quality wafers will have substantially uniform thickness and substantially uniform surface characteristics. In the semiconductor industry, wafers are used as a substrate supporting the fabrication of all types of semiconductor devices. Therefore, wafer quality can crucially influence the mechanical and/or electronic yield of wafer-based semiconductor device circuits. Accordingly, several techniques have been developed for wafer quality assessment and monitoring.
Fuel cells using proton exchange membranes (PEM) have gained considerable acceptance for automotive and portable power needs. It is expected that the fuel cell market will grow rapidly in the future. As this growth occurs, the fuel cell industry will also require monitoring techniques that are fast, non-destructive, and capable of high throughput. Such monitoring will be beneficial for membranes, electrode coatings, gas diffusion layers, others layers, surfaces or structures. The membrane properties that could be monitored include, but are not limited to, catalyst distribution and loading, electrode structure and porosity, thickness, membrane uniformity and defects, extent of curing, gas diffusion layer porosity, surface structure, and hydrophobicity.
Silicon wafers and fuel cell membranes are significantly different structures. Nonetheless, the monitoring of selected quality parameters in a continuous production environment would be advantageous for both wafer based device and fuel cell production.
A reflectometer may be used to measure selected physical characteristics of a semiconductor wafer or fuel cell membrane including but not limited to surface roughness, surface texture, crystalline grain orientation, antireflection coating thickness, wafer thickness, metallization characteristics and other parameters noted above. Reflectometer measurements are most typically used for wafer characterization before or after various steps in solar cell fabrication. In addition, a reflectometer can be used to measure certain membrane parameters, for example the thickness of fuel cell membranes or the detection and recognition of membrane defects. It is important to note however, that a typical reflectometer system is implemented in a static manner. Thus, a reflectometer based device is configured to measure one sample at a time. A typical reflectometer based system requires that a test wafer or a sample from a fuel cell membrane to be placed into an off-production line measurement chamber and then held stationary during optical examination. Thus, reflectometer based systems are best suited for process control where process monitoring is done by periodic sampling rather than continuous monitoring.
The solar cell fabrication industry is beginning to rely exclusively upon automated conveyor belt transport of wafers between processing steps. Generally, an array of wafers move through the processing apparatus more or less continuously leaving little or no opportunity for periodic sampling with a reflectometer. Thus, there exists a need to develop new continuous surface examination techniques to meet evolving industry requirements. Fuel cell membranes are also fabricated by performing various steps on a continuously moving web line. Thus, as the fuel cell industry grows, there is need to develop new techniques for monitoring selected membrane parameters on the web line itself, without stopping or slowing production.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.